So your argument is based on a few premises. Basic deductive logic suggests you simply need these all to be true. I think an analysis, like below is the best way to figure out what needs to be done.
The barrier to entry is Complexity
Unstated, but generally true.
The Shared Track should be Non-Complex
An unstated premise, but a reasonable one, is the track is not one of the more complicated tracks out there. Even with shared commonality, if the track isn't sufficiently simple, you will just see everyone making the same mistakes all of the time.
A Shared Track reduces choices
As long as the track isn't, say, Spellcasting, this is true.
Reducing Choices Reduces Complexity
This seems solid logic.
Commonality will reduce Complexity
The argument here is players will be able to help each other, and when one makes a mistake all can learn from it. However, it has another side that you have to be careful about: Players might end up with similar flawed assumptions. This may not be even visible because it leads to a continuous non-action.
Based on the above, we have a few criteria:
- The Shared Track should be non-complicated.
- The Shared Track needs to reduce choice more then it adds choice (IE, not spellcasting which is simple mechanically, but adds huge numbers of choices).
- The DM needs to be wary of Players making the same mistakes or having hidden flawed assumptions.
I would suggest, if there is an even number of PCs, Heroica.
- The Blade and the Bow establishes how Heroica works, and is good for anything that uses a weapon.
- Of Might and Guile gives a choice of abilities, both of which is quite useful to most builds. If the PCs paired off weapon users with non-weapon users, this results in Double crit range for weapon users, and a protective fire burst for non-weapon users.
- The Just and The Unjust Allows PCs to share items, making the item selection more interesting.
- The Truth and the Folly Is questionable how well it works on everyone.
- The Quick and the Dead makes it very difficult to hurt most group members.
- Aleph and Null Gives mobility and some basic healing
- With your Shield On It makes the group even more difficult to kill.
Other tracks that would work are also generally defensive in nature, unless you want to force all your PCs to be melee beasts, etc.